The present invention relates to a duplex stainless steel that is treated by an accelerated in-mold heat treatment treated after casting without using a separate heat treatment step. The duplex stainless steel has improved machinability and retains excellent corrosion resistant properties.
Rainger et al. (U.S. Pat. Nos. 4,612,069 and 4,740,254) describe a duplex stainless steel alloy having improved pitting resistance. The alloy described in those patents as "X-6" is herein called "Alloy 86". Alloy 86 is the result of adding 2 weight percent copper to an alloy (Alloy 75) without a simultaneous addition of molybdenum. The addition of copper without molybdenum allows the duplex stainless steel alloy to be very slowly control cooled in a tightly closed heat treatment furnace so that harmful tensile residual stresses are minimized while excellent ductility and corrosion resistance were retained.
A comparative commercially available molybdenum-containing alloy is 3RE60 SRG.RTM. from Avesta Prefab. A.V. of Sweden. Typical compositions of the duplex stainless steels discussed in this application are listed in Table I below in weight percent:
TABLE I ______________________________________ Alloy Cr Ni Cu Mo ______________________________________ Alloy 75 25.7 6.8 -- -- Alloy 86 26 6.8 2.0 -- X-11 26 6.8 2.0 -- 3RE60 SRG 18.5 5.0 -- 2.8 ______________________________________
Alloy 86 has useful applications in the chemical and pulp and paper manufacturing industries. The Alloy 86 can be used to make, but is not limited to, such products as vessels, retorts and piping; for paper machine roll shells such as coater rolls, grooved rolls and blind-drilled rolls; and for paper machine suction roll shell applications such as breast rolls, couch rolls, pickup rolls, press rolls and wringer rolls. These products require hundreds of hours of machining and hole-drilling time during their manufacture. The alloy X-11 of the present invention also has the same useful applications but with faster manufacturing cycle times and improved machinability and drillability.
Competitive pressures have directed metallurgical development towards duplex stainless steel alloys that have the necessary corrosion resistant properties for their end use, but can be manufactured in less time. The X-11 alloy has a desired combination of properties achieved through its chemical composition and accelerated in-mold heat treatment. Accelerated in-mold heat treatment manufacturing time by eliminating the separate heat treatment step needed by conventional alloys; by reducing machine tool setup with straighter, rounder centrifugal castings; by providing an alloy that is easier to machine and drill thereby reducing the amount of machining and drilling time needed to manufacture the product; and by reducing tool wear so that manufacturing equipment does not need to be stopped to change dull tools.
The required properties for the successful use of a duplex stainless steel alloy for suction roll shells in the pulp and paper making industries are a chemical composition that yields a duplex microstructure of austenite in a ferrite matrix, corrosion resistance in aggressive paper mill white waters, resistance to fatigue crack growth, and low residual stresses. In addition to its unique manufacturing properties, the X-11 alloy meets these service requirements.
Duplex stainless steels with intentional additions of molybdenum cannot be heat treated in the mold because the cooling rate is not fast enough to avoid the formation of embrittling and corrosion-degrading phases. An additional heat treatment step to dissolve those undesirable phases followed by a fast cooling step to prevent their reoccurrence is needed. The chemical compositions of Alloy 86 and X-11 with their copper addition for pitting resistance can tolerate much slower cooling rates and not form those brittle phases.
The machinability of duplex stainless steels is considered to be limited by their high annealed strength (Metals Handbook, Ninth Edition, pp. 689-690). Carlborg, C., Nilsson, A., and Franklind, P-A, "Machinability of Duplex Stainless Steel", Proceedings of a Conference Held in Beaune Bourgogne, France, October 1991, Vol. 1, pp. 683-696, discusses a variety of metallurgical variables such as high temperature strength, inclusions, structure and alloying elements on duplex stainless steel machinability but does not recognize the relationship of accelerated in-mold heat treatment for enhanced machinability. Charles, J., Dupoiron, F., Souglignac, P., and Gagnepain, Jr., "UR 35N Cu: A New Copper-Rich Molybdenum Free Duplex Stainless Steel with Improved Machinability, "Proceedings of a Conference Held in Beaune Bourgogne, France, October 1991, Vol. 2, pp. 1274-1281, reports that copper in a water-quenched duplex stainless steel improves machinability. However, the X-11 alloy at the same copper content as Alloy 86 has improved machinability as a result of accelerated in-mold heat treatment, which is not recognized by Charles et al.
The prior art of steel makers suggests that machinability of austenitic stainless steels can be enhanced by additions of alloying elements such as sulfur and selenium that may reduce corrosion performance (Metals Handbook Ninth Edition p. 686). Or, special steel making practices need to be implemented to control oxide inclusion composition (Metals Handbook Ninth Edition p. 688; Johansson, R., Davison, R., "Wrought Duplex Stainless Steel Suction Rolls With High Performance", 1996 TAPPI Engineering Conference Proceedings, pp. 103-109; Carsson, T., "Prodec-How to Solve Machining Problems", pp. 9-12). Neither practice is required for the X-11 alloy to have enhanced machinability and drillability.